In cold blood

Q From Andy Cilley: What is the origin of in cold blood?

A Think of the effect of doing something with emotion, passion or great exertion. The blood flows to your face and you feel hot. At a time before our understanding of the human body was as good as it is now, it was thought that the blood actually grew hot at such times. We still have a set of phrases in the language that reflect this, such as his blood boiled or in the heat of passion, which contrast with others that describe a person whose blood is cold or cool, so detached or uninvolved. So, an action that was carried out without excitement or emotional involvement was said to have been taken when the blood was cold (the exact equivalent of the French sang froid). The phrase is now usually taken to refer to some act that might look like an act of passion but which was actually done with cool deliberation; it’s first recorded in Joseph Addison’s Spectator in 1711.